Saturday, February 24, 2007

Tuples are a handy construct found in many programming languages. Oddly enough, they are lacking in mainstream languages like Java, C#, C++ etc.

Java was actually designed to have tuples from the start, but they never quite got in. At one point, I tried to add a tuple-like construct as part of JSR-65. We wanted to extend array initializers to full blown expressions. Eventually, that effort got quashed; I didn’t really resist, since the extension was painful, as such things always are in Java. We should have just done straight tuples as a separate construct - but at the time, that was frowned upon too.

Tuples are useful for a variety of reasons. If you need to package several objects together, but don’t want to define a class just for that purpose,you can use a tuple. This subsumes features like “multiple-value returns” which people have been seeking in Java for many years (without success). It also pretty much covers the need for variable-arity methods. Java went with a special sugar for these instead, a decision I initially opposed; I was eventually convinced to cave in on this, which I really regret (the responsible parties know who they are).

Anyway, that’s all water (well, coffee actually) under the bridge. Back to the main point.

Once you have tuples, handy uses crop up frequently, and you wonder how you ever got along without them.Smalltalk doesn’t quite have tuples. Instead it has array literals, which are compile time constants and so rather limiting. Squeak does have tuples, though they are not very often used. It would be best to forego array literals entirely and replace them with tuples.

There are subtleties. Literal tuples are best defined as read only. One reason for this is that readonly tuples are more polymorphic. Long tuples are subtypes of short ones:

{S. T. U. V } <= {S. T. U} <= {S. T} <= {S}

Read only tuples are covariant:

T1 <= T2, S1 <= S2 ==> {S1. T1} <= {S2. T2}

And a read-only literal tuple can be viewed as a list:

S <= U, T <= U, s: S , t : T ==> {s. t} : List[U]

where List[E] is a generic (note I'm using square brackets for type parameters) readonly type for lists (such as SeqCltn[E] in Strongtalk). It should be clear that it is very important to relate tuples to the general collection hierarchy.Note that it is unsound to assume that

S <= U, T <= U ==> {S. T} <= List[U]

since

{S. T. V} <= {S.T} for all V, but if V <= U does not hold, {S. T. V} <= List[U] does not hold either.

Now if you want writable tuples, you can use an idiom like{e1. e2. e3} asWritableTuple, which should create a copy of the literal that is writable (don’t worry about the cost of the copy; let the system figure it out).